This invention relates to disposable, thin-wall, plastic lids and more particularly comprises a new and improved lid having a stacking facility which cooperates with other similar lids to form a very stable lid stack and to prevent jamming when axial loads are applied to them.
Recent years have witnessed the rapid growth of the use of disposable thermoformed plastic container lids, particularly in connection with fast food establishments, vending machines, automatic container filling equipment and the like. Considerable efforts have been directed to the design of such lids to include, as an integral part of the lid, a stacking facility by which a plurality of such lids may be nested one on top of another, in a stack, but in a manner which precludes them from becoming jammed when the stack is subjected to an axially applied load.
The most common type of stacking facility employed in connection with container lids is incorporated into and is made part of the depending peripheral skirt of the lid. Typically, such stacking facility is in the form of an undercut region between the top and bottom of the skirt which defines an inwardly extending stacking ring of slightly reduced diameter, which creates a positive interference with the upper outer peripheral region of the next lower lid in the stack. There are certain disadvantages, however, to undercut stacking rings of that type. For example, the undercuts make it difficult to remove the articles from the mold cavities after they are formed, and special stripping mechanisms frequently are necessary to eject the articles from the cavities. Difficulties are also encountered in actually forming the details of the undercuts, and if the details are not formed well, the articles may jam together when an axial load is applied. In addition, it may be noted that because of the typical thermoforming procedures employed to manufacture such lids, the lid skirts are somewhat thinner than the other portions of the lids and consequently are of reduced rigidity. As a result, the typical undercut stacking ring in the lid skirt does not always perform well in that it may sometimes flex and distort under an axial load and become jammed with the next lower lid in the stack.
The foregoing difficulties have not gone unrecognized and efforts have been made to avoid them. For example, suggestions have been made to provide the lid with a stacking facility in the form of projections extending downwardly from the closure wall of the lid to rest on the upper surface of the closure wall of the next underlying lid in the stack. This type of construction, however, presents other difficulties. For example, the downwardly extending projections may extend into food packaged in the container, which is undesirable.
In addition to the above difficulties the prior art stacking facilities of the general character discussed above do not create a very stable stack. Even though the stacking facilities may tend to prevent axial jamming, they do not dependably maintain the lids in vertical alignment; rather, they permit the lids to slip sideways to create a rather sloppy pile of lids when the lids are not confined in a magazine or some special storage container. This is particularly troublesome when the lids are designed for over-the-counter use in fast food establishments where they are normally placed on a counter without any magazine or storage container about them so as to be readily accessible to those working at the counter.
It is among the primary objects of the invention to provide an improved container lid construction which avoids the foregoing and other difficulties.